Scientists have created the first map that shows how the Greenland Ice Sheet has moved over time, revealing that ice in the interior is moving more slowly toward the edges than it has, on average, during the past 9,000 years. The findings, which researchers said don’t change the fact that the ice sheet is losing mass overall and contributing to sea level rise, are published in the Feb. 5 issue of Science. Along Greenland’s periphery, many glaciers are rapidly thinning. However, the vast interior of Greenland is slowly thickening, a process the new study clarifies.

“Scientists are very interested in understanding how ice sheets flow and how that flow may have been different in the past. Our paleo-velocity map for Greenland allows us to assess the flow of the ice sheet right now in the context of the last several thousand years,” said lead author Joe MacGregor of The University of Texas at Austin’s Institute for Geophysics (UTIG), a research unit of the Jackson School of Geosciences. The study builds on earlier UTIG-led research that developed a database of the many layers within Greenland’s ice sheet. Using this database, the scientists determined the flow pattern for the past 9,000 years — in effect creating a “paleo-velocity” map.

The authors identified three causes for this deceleration. First is that snowfall rates were generally higher during the past 9,000 years, second is the slow stiffening of the ice sheet over time, and third is the collapse of an “ice bridge” that used to connect Greenland’s ice to that on nearby Ellesmere Island. Of most interest were the last two. “Like many others, I had in mind the ongoing dramatic retreat and speedup along the edges of the ice sheet, so I’d assumed that the interior was faster now too. But it wasn’t,” said MacGregor.In comparing the paleo-velocity map with modern flow rates, researchers found that the ice sheet’s interior is moving more slowly now than during most of the Holocene, a geological period that began about 11,700 years ago and runs to the present.

“The ice that formed from snow that fell in Greenland during the last ice age is about three times softer than the ice being formed today,” according to William Colgan of York University’s Lassonde School of Engineering, a co-author of the study. Because of this difference, the ice sheet is slowly becoming stiffer. As a consequence, the ice sheet is flowing more slowly and getting thicker over time. This effect is most important in southern Greenland, where higher snowfall rates have led to rapid replacement of ice from the last glacial period with more modern Holocene ice. “But that didn’t explain what was happening elsewhere in Greenland, particularly the northwest, where there isn’t as much snowfall, so the stiffening effect isn’t as important,” said MacGregor.

The explanation of deceleration in the northwest lies in the collapse 10,000 years ago of an “ice bridge” across Nares Strait, which used to connect Greenland’s ice to that on Ellesmere Island. The collapse of the ice bridge at the end of the last ice age led to acceleration in the northwest, but the ice sheet has since returned to a slower pace.

These changes, which started thousands of years ago, affect our understanding of the changing Greenland Ice Sheet even today. Scientists often use GPS and altimeters aboard satellites to measure the elevation of the ice surface and study how much mass is being lost or gained across the ice sheet. When correcting for other known effects on the surface elevation, any leftover thickening is assumed to be due to increasing snowfall, but this study shows that may not be the case. “We’re saying that recent increases in snowfall do not necessarily explain present-day interior thickening,” said Colgan. “If you’re using a satellite altimeter to figure out how much mass Greenland is losing, you’re going to get the answer slightly wrong unless you account for these very long-term signals that are evident in its interior.”

Scientists studying data from the top of the Greenland ice sheet have discovered that during winter in the center of the world’s largest island, temperature inversions and other low-level atmospheric phenomena effectively isolate the ice surface from the atmosphere — recycling water vapor and halting the loss or gain of ice. A team of climate scientists made the surprising discovery from three years of data collected at Summit Camp, an arid, glaciated landscape 10,500 feet above sea level in the middle of the Greenland ice sheet. “This is a place, unlike the rest of the ice sheet, where ice is accumulating,” says Max Berkelhammer, assistant professor of earth and environmental sciences at the University of Illinois at Chicago. Berkelhammer is first author on the study, reported in Science Advances, an open-access online publication of the journal Science. Near Greenland’s coasts, Berkelhammer said, “it’s relatively warm, and the ice melts faster and faster.”

“But in the center of the ice sheet, it’s 25 below zero Celsius (-13 F), so it’s always freezing, even if it warms. It’s a very rare occurrence to go above freezing,” he said. The authors note that “despite rapid melting in the coastal regions of the ice sheet, a significant area — approximately 40 percent — rarely experiences surface melting.” Solid ice can be lost not only by melting into liquid water. Under certain conditions, it can vaporize by sublimation, a one-step transition from solid to gas. Such conditions exist at the high-altitude, dry, frigid surface of Greenland’s interior. “Sublimation is common there, unlike other places,” Berkelhammer said. “We looked at the exchange of water between the ice sheet and the air above it through condensation, evaporation, and sublimation.”

At Summit Camp, a 150-foot tower was used to draw air samples at various heights above the surface and pipe the air into a laboratory buried a few feet below the ice. Lasers analyzed the air for two different isotopes of oxygen in H2O, whose ratio indicates the temperature at which the water molecules became airborne. “We noticed a specific process that was occurring, where low-level fog would form right above the surface of the ice sheet,” Berkelhammer said. A fogbow – a rainbow caused by fog – often appeared. “As ice sublimates from the surface, it forms a fog,” he said. “As the particles get heavier and settle back to the surface, you get recycling, rather than dissipation that would remove ice.” In winter, 80 percent of the ice that would otherwise be lost is recycled, Berkelhammer said. “So it’s an incredibly efficient process.”

But many questions remain as to how this boundary-layer recycling contributes to models of climate change. We expected sublimation to increase with temperature, but we find no net loss” of ice over time, Berkelhammer said, again referring just to the interior of the ice mass. “You could say, if this process changes, you’d lose ice significantly faster. Or, if (recycling) becomes even more efficient, you would conserve even more ice mass. “We can’t predict,” he said. “And we don’t know from the ice-core records what the history is.” The next step, he said, is to run experiments to see how sublimation changes with temperature associated with past and future changes in atmospheric carbon dioxide levels, to see how recycling fits into climate models.

“If we want to model how the ice sheet is warming, we need to include everything we know,” he said. “This is a new process to incorporate in models.” But Berkelhammer cautions against over-interpreting the recycling as good news for the ice sheet or the planet, as its overall effect is likely to be relatively minor. “This is small potatoes compared to the calving that’s going on along the coasts,” he said. “Every time we go back to Greenland, the edge of the ice is farther away from the coast.”

Top: The total daily contribution to the surface mass balance from the entire ice sheet (blue line, Gt/day). Bottom: The accumulated surface mass balance from September 1st to now (blue line, Gt) and the season 2011-12 (red) which had very high summer melt in Greenland. For comparison, the mean curve from the period 1990-2013 is shown (dark grey). The same calendar day in each of the 24 years (in the period 1990-2013) will have its own value. These differences from year to year are illustrated by the light grey band. For each calendar day, however, the lowest and highest values of the 24 years have been left out.